This disclosure relates to pressure relief valves, and more particularly, to pressure relief valves suitable for use in cryogenic applications.
Typically, a pressure relief valve may be used to control or limit pressure in a system or vessel, which can build up by, for example, a process upset, instrument or equipment failure, or fire. In some instances, pressure relief valves may be used in a chemical processing plant that distills natural gas into pure methane, e.g., a “methanizer.” The process eliminates impurities like toluene and ethane from the natural gas to provide pure methane. The distillation is done by the adiabatic expansion of natural gas. Pressurized natural gas is made to work on compressors. The reduction in pressure to perform work is reflected by a drop in temperature. At different stages of temperature drops, the different compounds, such as toluene and ethane, start distilling and are collected in separate containers. Since methane has the lowest atomic weight in the gases comprising natural gas, pure methane results at the final stage of distillation.
The process fluid coming out of the methanizer (e.g., methane) is in liquid form and may also be used elsewhere in the plant or sold as liquefied natural gas (“LNG”). A safety, or pressure, relief valve may be used to protect against overpressures at the methanizer output, or any other location within the piping system. LNG facilities may operate in the range of −150 degrees F. to −450 degrees F.
In some instances, premature and/or undesirable leaks from prior art pressure relief valves may occur due in part to a difference in temperature between a fluid flowing to an inlet of the pressure relief valve and an ambient condition within the valve. Such a leak may begin as a microleak but, without attention, may increase in flow to a macroleak, thereby preventing the valve from maintaining a desired pressure-seal in the system. Very large temperature differences may occur with valves in cryogenic service. In some instances, the temperature differences between the fluid flowing to the valve inlet and the ambient valve condition may be in the range of 250 degrees F. Referring briefly to FIGS. 7A and 7B, sectional views of a conventional prior art disc 700 of a prior art pressure relief valve used in a high temperature (e.g., steam) application is illustrated. Such prior art discs have been used in high temperature applications for over 50 years. The disc 700 includes lips 705 extending from a bottom surface of the disc 700 and directed towards a centerline of the disc 700. FIG. 6 shows the disc 700 in a non-operational state, i.e., with no high temperature fluid flowing through the valve in which the disc 700 is placed. Upon introduction of the high temperature fluid to the valve and opening of the valve, a temperature gradient occurs across the lips 705 (e.g., high temperature at an inlet of the valve compared to ambient temperature at an outlet of the valve). FIG. 7B illustrates a deflection of the lips 705 due to the temperature gradient and thermal characteristics of the disc 700. As illustrated, due to a high temperature fluid, the lips 705 deflect in the direction “X.” Such deflection may help seal the valve (i.e., cause the disc to seat on a nozzle in the valve) to leaks, such as microleaks and/or macroleaks.
In some instances, a prior art pressure relief valve disc such as disc 135 (as shown in FIGS. 1A, 1B and 1C) experiences micro or macroleaks when such a valve is placed in cryogenic service. Such prior art valve discs may also experience galling. In some cases, galling is a form of surface damage on an interior surface of the pressure relief valve arising between sliding solids. Galling is, typically, distinct from damage caused by microscopic (usually localized) roughening and creation of protrusions (i.e., lumps) above the interior surface. Galling may contribute or exacerbate the leaks experienced by the pressure relief valve due to the temperature difference experienced in cryogenic service.
Therefore, there has been long felt and unmet need for a unique design for discs for relief valves used in cryogenic service that solve the problems discussed above.